Internet Draft                                                M. Elvey
Document: draft-ietf-sieve-refuse-reject-01     The Elvey Partnership,
Expires: September 2006                                    A. Melnikov
                                                             Isode Ltd
                                                            March 2006

   The SIEVE mail filtering language - reject and refuse extensions

Status of this Memo

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   This memo defines the SIEVE mail filtering language (RFC
   <<3028bis>>) "reject" extension.

   A Joe-job is a spam run forged to appear as though it came from an
   innocent party, who is then generally flooded by the bounces,
   Message Disposition Notifications (MDNs) and messages with
   complaints.  The original Sieve "reject" action defined in RFC 3028
   required use of MDNs for rejecting messages, thus contributing to
   the flood of Joe-job spam to victims of Joe-jobs.  This document
   updates definition of "reject" to allow for rejecting messages
   during the SMTP transaction.

Table of Contents

  1. Introduction                                                   3
  2. Conventions Used in this Document                              3
  3. SIEVE "reject" extension                                       4
     3.1 Action reject                                              4
     3.2 "reject" compatibility with other actions                  7
  4. Security Considerations                                        7
  5. IANA Considerations                                            7
     5.1 reject extension registration                              7
     5.2 refuse extension registration                              8
  6. References                                                     8
     6.1 Normative References                                       8
     6.2 Informative References                                     8
  7. Acknowledgments                                                8
  8. Author's Addresses                                             9
  9. Intellectual Property Rights Statement                         9
  10. Full Copyright Statement                                     10
  11. Changes from RFC 3028                                        10
  12. Change Log                                                   10

1.   Introduction

   The SIEVE mail filtering language [SIEVE] "reject" action defined
   in RFC 3028 only allows users to refuse delivery of a message by
   sending an [MDN].

   This document updates definition of the "reject" action to permit
   users to handle unwanted email in a way that is sometimes
   preferable to the existing 'discard' and the original 'reject'
   capabilities.  When a spam-detection system suspects a message is
   spam, but isn't certain, discarding the email is considered too
   risky for some users, for example, those who receive sales leads by
   email. They are willing to use the reject command. Users are
   willing to reject but not discard because the sender of an email
   incorrectly marked as spam will receive a notification that the
   email was refused, and will likely try again to contact the
   intended recipient, perhaps via another method of communication.
   Unfortunately, this usage is problematic, because in the usual
   case, the email is indeed spam, and the alleged sender to whom the
   MDN caused by the reject will be sent will often be an innocent Joe-
   job victim. The updated "reject" is less likely to result in email
   to an innocent victim, because it allows to refuse to accept an
   email for delivery instead of accepting it and then sending an MDN.
   Much spam is sent through open proxies, so SMTP level refusal
   reduces Joe-job bounces (AKA backscatter) resulting from usage of
   MDNs. The updated "reject" will also reduce Joe-jobs caused by
   virus self-propagation via emails with false sender information.
   SMTP level refusal may conserve bandwidth, by reducing the number
   of MDNs sent. Further discussion highlighting the risks of
   generating MDNs and the benefits of protocol level refusal can be
   found in [Joe-DoS].

2.   Conventions Used in this Document

   Conventions for notations are as in [SIEVE] section 1.1, including
   use of [KEYWORDS].

   This document does not attempt to define what exactly constitutes a
   spam or virus containing email or how it should be identified, or
   what actions should be taken when detected.

3.   SIEVE "reject" extension

   SIEVE implementations that implement the "reject" action must use
   the "reject" capability string.

3.1  Action reject

      Usage:   reject <reason: string>

   The "reject" action cancels the implicit keep and refuses delivery
   of a message. How message is refused depends on capabilities of
   mail component (MUA, MDA or MTA) executing the Sieve script. The
   Sieve interpreter must do one of the following actions, as detailed
   by the following priority table (items listed earlier take
   precedence). Note that if action can not be taken or fails, the
   interpreter should try the next item in the list:

     1.   If message return-path (MAIL FROM) is empty the message MAY be
          accepted and discarded.
     2.   If a "reject" implementation performs a return-path
          verification and it clearly indicates that the message has a
          forged return-path, the implementation need not refuse mail
          delivery, but rather MAY accept and discard it.
     3.   Message delivery is refused by sending 5XX response code over
          SMTP/LMTP. See section 3.1.1 for more details.
     4.   Message delivery is refused by sending a non delivery report
          (DSN). See section 3.1.2 for more details.
     5.   Message delivery is refused by sending a message disposition
          notification report (MDN). See section 3.1.3 for more details.

3.1.1 Rejecting message at SMTP/LMTP protocol level

   Sieve engines that are able to reject messages at SMTP/LMTP level
   SHOULD use 550 response code. Note that it is not always possible
   to do that, for example if the message is arriving over SMTP and
   has multiple recipients, some of which have accepted the message.
   See section 3.1.2 for recommendations on how to reject message in
   such case.

   <<Open issue: do we want to allow for non-ascii text below and do
   we need a way to control "reject with DSN containing non-ascii
   text" versa "replace non-ascii characters with ?">>
   Note that SMTP [SMTP] doesn't allow for non-ASCII characters in
   SMTP response text. It is an error for non-ASCII characters to
   appear in the "reason" string (unless the client and the server use
   an SMTP extension that allows for transmission of non-ASCII reply

   If the "reason" string is multiline, than the reason text MUST be
   returned as a multiline SMTP/LMTP response, per [SMTP], section
   4.2.1. Any line MUST NOT exceed the SMTP limit on the maximal line
   length. To make the reason string conform to any such limits the
   server MAY insert CRLFs and turn the response into multiline

   In the following script (which assumes support for the spamtest and
   fileinto extensions), messages that test highly positive for spam
   are refused.

      require ["reject", "spamtest",
      "comparator-i;ascii-numeric", "fileinto"]

      if spamtest :value "ge" :comparator "i;ascii-numeric" "6" {
                   refuse text:
   AntiSpam engine thinks your message is spam.
   It is therefore being refused.
   Please call 1-900-PAY-US if you want to reach us.
      } elsif spamtest :value "ge" :comparator "i;ascii-numeric" "4" {
                   fileinto "Suspect";

   The following excerpt from an SMTP session shows it in action.
   C: DATA
   S: 354 Send message, ending in CRLF.CRLF.
   C: .
   S: 550-AntiSpam engine thinks your message is spam.
   S: 550-It is therefore being refused.
   S: 550 Please call 1-900-PAY-US if you want to reach us.

   If the SMTP/LMTP server supports RFC 2034 [ENHANCED-CODES] it MUST
   prepend an appropriate Enhanced Error Code to the "reason" text.
   Enhanced Error code 5.7.1 or a more generic 5.7.0 are RECOMMENDED.
   With Enhanced Error Code the response to DATA command in the SMTP
   example below will look like:

     S: 550-5.7.1 AntiSpam engine thinks your message is spam.
     S: 550-5.7.1 It is therefore being refused.
     S: 550 5.7.1 Please call 1-900-PAY-US if you want to reach us.

   if the server selected "5.7.1" as appropriate.

3.1.2 Rejecting message by sending DSN

   If the implementation receives a message via SMTP that has more
   than one RCPT TO that has been accepted by the server, and at least
   one but not all of them are refusing delivery (whether the refusal
   is caused by execution of a Sieve "reject" or for another reason).
   In this case, the server MUST accept the message and generate DSNs
   for all recipients that are refusing it. Note that this exception
   does not apply to LMTP, as LMTP is able to reject messages on a per-
   recipient basis.

3.1.3 Rejecting message by sending MDN

   When Sieve engine is running inside MUA it has no ability to reject
   the message before it was delivered, as the message is already
   deliverd. In this case the client should send a Message Disposition
   Notification [MDN] back to the sender. It resends the message to
   the sender as specified in the Return-Path header field, wrapping
   it in a "reject" form, noting that it was rejected by the
   recipient.  In the following script, a message is rejected and
   returned to the sender.

                require ["reject"]

                if header :contains "from" ""
                reject text:
   I am not taking mail from you, and I don't
   want your birdseed, either!"

   A reject message MUST take the form of a failure MDN as specified
   by [MDN]. The human-readable portion of the message, the first
   component of the MDN, contains the human readable message
   describing the error, and it SHOULD contain additional text
   alerting the original sender that mail was refused by a filter.
   This part of the MDN might appear as follows:

   The message was refused by the recipient's mail filtering program.
   The reason
   given was as follows:

   I am not taking mail from you, and I don't want your birdseed,

   The MDN action-value field as defined in the MDN specification MUST
   be "deleted" and MUST have the MDN-sent-automatically and automatic-
   action modes set.

3.2  "reject" compatibility with other actions

   A "reject" action cancels the implicit keep.

   Implementations MUST prohibit the execution of more than one reject
   in a SIEVE script. "Reject" is also incompatible with the
   "vacation" [VACATION] extensions.

   Any action that would modify the message body will not have effect
   on the body of any message refused by "reject" using the 550 SMTP
   response code <<and might not have any effect on context of
   generated DSN/MDNs>>.

   Implementations SHOULD prohibit reject when used with other

4.   Security Considerations

   The "reject" extension does not raise any security considerations
   that are not present in the base [SIEVE] protocol, and these issues
   are discussed in [SIEVE]. <<Mail loops>>

   The Introduction section talks about why rejecting messages before
   delivery is better then accepting and bouncing them.

5.   IANA Considerations

   The following section provides the IANA registrations for the Sieve
   extensions specified in this document:

5.1  reject extension registration

   IANA is requested to update the registration for the SIEVE "reject"
   extension to point to this document.
   IANA is also requested to update Tim Showalter's email address to

5.2  refuse extension registration

   IANA is requested to remove registration of the refuse extension.

6.   References

6.1  Normative References

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, March 1997.

   [SIEVE] Showalter, T. and P. Guenther, "Sieve: An Email Filtering
   Language", Work-in-progress, draft-ietf-sieve-3028bis-XX.txt

   [SMTP] Klensin, J. (Editor), "Simple Mail Transfer Protocol", AT&T
   Laboratories, RFC 2821, April 2001.

   [LMTP] Myers, J., "Local Mail Transfer Protocol", Carnegie-Mellon
   University, RFC 2033, October 1996.

   [DSN] Moore , K., Vaudreuil, G., "An Extensible Message Format for
   Delivery Status Notifications", University of Tennessee, Lucent
   Technologies, RFC 3464, January 2003.

   [MDN] Fajman, R., "An Extensible Message Format for Message
   Disposition Notifications", National Institutes of Health, RFC
   2298, March 1998.

   [ENHANCED-CODES] Freed, N., "SMTP Service Extension for Returning
   Enhanced Error Codes", Innosoft, RFC 2034, October 1996.

6.2  Informative References

   [Joe-DoS] Stefan Frei, Ivo Silvestri, Gunter Ollmann, "Mail Non
   Delivery Message DDoS Attacks", 5 April 2004",

   [SPAMTEST] Daboo, C., "SIEVE Email Filtering: Spamtest and
   Virustest Extensions", work in progress, draft-ietf-sieve-
   <<Note that this reference can be safely replaced with RFC 3685.>>

7.   Acknowledgments

   Thanks to Ned Freed, Cyrus Daboo, Arnt Gulbrandsen, Kristin Hubner,
   Mark E. Mallett, Philip Guenther and Michael Haardt for comments
   and corrections.

   The authors gratefully acknowledge the extensive work of Tim
   Showalter as the author of the RFC 3028, which originally defined

8.   Author's Addresses

   Matthew Elvey
   The Elvey Partnership, LLC
   3042 Sacramento-ietf St Ste 04
   San Francisco, CA


   Alexey Melnikov
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex, TW12 2BX


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10.  Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
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   This document and the information contained herein are provided on


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11.  Changes from RFC 3028

  Clarified that the "reject" action cancels the implicit keep.
  Extended list of allowable actions on reject to include protocol
  level message rejection and generation of DSNs.

12.  Change Log

  <<Note that this section will be deleted before publication.>>

   00   First formal draft.
   01   Explicit RFC 2034 support, disallow "refuse" in MUAs, typos
        corrected, clarifications, etc.
   02   Many insubstantial editorial changes (mostly rewording text for
        readability). Added text regarding non-ASCII characters in the
        refuse "reason" string. Added an exception allowing return-path
        forgery to justify discarding a message.
   03   (Renamed to be SIEVE WG 00) - Updated boilerplate, added reject
        action from the base spec, acknowledged Tim as the author of
   04   (SIEVE WG 01) Based on WGLC feedback, the refuse and the reject
        actions were merged into a single action called reject. Text
        reorganized as the result. Typos and examples corrected. Updated
        IANA registration and Security Considerations sections.